Greymouth was not far behind late last week, climbing to 18.8degC, while Westport topped 19degC.
Hokitika weather observer Mark Crompton said the 19.7degC was a record for June, topping the previous one of 18.6degC in 2014 by 1.1degC.
Greymouth Nursery owner John Phillips has had reports of spring flowers such as freesias blooming.
"It seems untimely, very early. They have been lulled into a false sense of security."
The forecast cold weather this weekend would slow down the early blooming, he said.
Aratika sheep farmer Dianne Milne said despite warmer days this past week, she was not expecting to lamb until later this month.
Last month, during the cold snap, they had 17 frosts in the Arnold Valley, near Lake Brunner, and with no rain for three weeks, country folk were keeping an eye on their water tanks, she said.
Federated Farmers West Coast meat and wool chairman Simon Cameron, who farms at Haast, said after the warmer weather, the worst thing would be to get a cold, wet spring.
However, the mild weather meant good grass growth, and it was firm underfoot.
"It certainly helps farming conditions."
Reefton had been hit with two weeks of fogs and frosts early last month.
However, it managed to get to 14degC on Tuesday last week and 13degC the next day, which was warm for Reefton in winter, weather observer Tony Fortune said.
"Thirteen is high for us."
Niwa said it had been an abnormally warm start to winter in much of New Zealand.
However, as July starts, cold air is forecast to move in from the Antarctic shelf.
By tomorrow, the West Coast could be looking at frosts again.
The skifields are hoping this weekend delivers the long-awaited snowfall.
Porters Skifield postponed its opening on Wednesday, Mount Cheeseman is watching to see what the weekend brings and Craigieburn is due to open on July 12 — snow permitting.
